I. Field of the Invention
The present invention concerns the fields of molecular medicine and virology. More specifically, the present invention relates to compositions and methods for site specific retroviral integration of therapeutic nucleic acids.
II. Description of Related Art
A prerequisite to persistent gene expression from a lentiviral vector is integration into the chromosome of a transduced cell. This property can be exploited for the long-term correction of genetic diseases; however, the integration reaction carries a potential for mutagenesis. Thus, the nonspecific nature of integration presents a potential drawback for introducing a transgene with lentiviral or other integrating vectors. Insertional mutagenesis may disrupt normal cell actions by inactivating an essential host gene or inappropriately causing overexpression of an undesirable gene. Recently, 3 of 11 children with X-linked SCID treated with ex vivo MuLV retroviral gene transfer of the IL-2 common γ chain into CD34+ cells (Cavazzana-Calvo et al., 2000; Hacein-Bey-Abina et al., 2002) developed a T cell leukemia-like illness, in 2 cases possibly related to a single insertional event in one LMO2 allele (Hacein-Bey-Abina et al., 2003). This incident has raised important issues that must be considered if integrating vectors are to be developed for somatic cell gene therapies.
Bushman and co-workers investigated the chromosomal targets for integration of HIV-1 and an HIV-based vector in a human T cell line (Schroder et al., 2002). The availability of the draft sequence of the human genome has aided the analysis of integration sites. A total of 524 sites of HIV integrations were mapped. It was discovered that transcriptionally active genes were favored as sites of integration. Additionally, hot spots for integration were noted within the genome, including one 2.4 kb region that contained 1% of all integration events (Schroder et al., 2002). Perhaps DNA may be more accessible to the vector pre-integration complex in transcriptionally active areas of the genome; alternatively, the transcription factors bound to trans-acting elements in promoters may interact with the pre-integration complex. In contrast, integration may be less likely in the more tightly constrained noncoding regions or near transcriptionally inactive genes. Furthermore, recent studies have found that the MLV vector has a different integration preference and favors integration in transcriptional start regions (Wu et al., 2003).
Given the described difficulties, additional compositions and methods are needed to develop a more controlled integration of DNA into the genome of a cell for therapy.